1. Field of the Invention
The present invention relates to the papermaking arts. More specifically, the present invention relates to forming fabrics for the forming section of a paper machine.
2. Description of the Prior Art
During the papermaking process, a cellulosic fibrous web is formed by depositing a fibrous slurry, that is, an aqueous dispersion of cellulose fibers, onto a moving forming fabric in the forming section of a paper machine. A large amount of water is drained from the slurry through the forming fabric, leaving the cellulosic fibrous web on the surface of the forming fabric.
The newly formed cellulosic fibrous web proceeds from the forming section to a press section, which includes a series of press nips. The cellulosic fibrous web passes through the press nips supported by a press fabric, or, as is often the case, between two such press fabrics. In the press nips, the cellulosic fibrous web is subjected to compressive forces which squeeze water therefrom, and which adhere the cellulosic fibers in the web to one another to turn the cellulosic fibrous web into a paper sheet. The water is accepted by the press fabric or fabrics and, ideally, does not return to the paper sheet.
It should be appreciated that the forming, press and dryer fabrics all take the form of endless loops on the paper machine and function in the manner of conveyors. It should further be appreciated that paper manufacture is a continuous process which proceeds at considerable speeds. That is to say, the fibrous slurry is continuously deposited onto the forming fabric in the forming section, while a newly manufactured paper sheet is continuously wound onto rolls after it exits from the dryer section.
Press fabrics also participate in the finishing of the surface of the paper sheet. That is, press fabrics are designed to have smooth surfaces and uniformly resilient structures, so that, in the course of passing through the press nips, a smooth, mark-free surface is imparted to the paper.
Press fabrics accept the large quantities of water extracted from the wet paper in the press nip. In order to fill this function, there literally must be space, commonly referred to as void volume, within the press fabric for the water to go, and the fabric must have adequate permeability to water for its entire useful life. Finally, press fabrics must be able to prevent the water accepted from the wet paper from returning to and rewetting the paper upon exit from the press nip.
The paper sheet finally proceeds to a dryer section, which includes at least one series of rotatable dryer drums or cylinders, which are internally heated by steam. The newly formed paper sheet is directed in a serpentine path sequentially around each in the series of drums by a dryer fabric, which holds the paper sheet closely against the surfaces of the drums. The heated drums reduce the water content of the paper sheet to a desirable level through evaporation.
Those skilled in the art will appreciate that fabrics are created by weaving, and have a weave pattern which repeats in both the warp or machine direction (MD) and the weft or cross-machine direction (CD). Woven fabrics take many different forms. For example, they may be woven endless, or flat woven and subsequently rendered into endless form with a seam. It will also be appreciated that the resulting fabric must be uniform in appearance; that is there are no abrupt changes in the weave pattern to result in undesirable characteristics in the formed paper sheet. Due to the repeating nature of the weave patterns, a common fabric deficiency is a characteristic diagonal pattern in the fabric. In addition, any pattern marking imparted to the formed tissue will impact the characteristics of the paper.
The present invention may relate specifically to the forming fabrics used in the forming section. Forming fabrics play a critical role during the paper manufacturing process. One of its functions, as implied above, is to form and convey the paper product being manufactured to the press section.
However, forming fabrics also need to address water removal and sheet formation issues. That is, forming fabrics are designed to allow water to pass through (i.e. control the rate of drainage) while at the same time prevent fiber and other solids from passing through with the water. If drainage occurs too rapidly or too slowly, the sheet quality and machine efficiency suffers. To control drainage, the space within the forming fabric for the water to drain, commonly referred to as void volume, must be properly designed.
Contemporary forming fabrics are produced in a wide variety of styles designed to meet the requirements of the paper machines on which they are installed for the paper grades being manufactured. Generally, they comprise a base fabric woven from monofilament and may be single-layered or multi-layered. The yarns are typically extruded from any one of several synthetic polymeric resins, such as polyamide and polyester resins, used for this purpose by those of ordinary skill in the paper machine clothing arts.
The properties of absorbency, strength, softness, and aesthetic appearance are important for many products when used for their intended purpose, particularly when the fibrous cellulosic products are facial or toilet tissue, paper towels, sanitary napkins or diapers.
To generate bulk, cross directional tensile, absorbency, and softness in a sheet of paper, a fabric will often be constructed so that the top surface exhibits topographical variations. These topographical variations are often measured as plane differences between strands. For example, a plane difference is typically measured as the difference in height between two adjacent weft (cross direction) strands in the plane of the wear side surface or as the difference in height between MD knuckles and CD knuckles in the forming surface. Bulk, cross directional tensile, absorbency, and softness are particularly important characteristics when producing sheets of tissue, napkin, and towel paper. Hence, tissue forming fabrics preferably exhibit plane differences in the forming side.
The design of forming fabrics additionally involves a compromise between the desired fiber support and fabric stability. A fine mesh fabric may provide the desired paper surface and fiber support properties, but such design may lack the desired stability resulting in a short fabric life. By contrast, coarse mesh fabrics provide stability and long life at the expense of fiber support and the potential for marking. To minimize the design tradeoff and optimize both support and stability, multi-layer fabrics were developed. For example, in double and triple layer fabrics, the forming side is designed for sheet and fiber support while the wear side is designed for stability, void volume, and wear resistance.
Double layer fabrics are commonly used within the paper industry. A typical double layer fabric comprises a set of forming weft yarns (shutes) and a set of wear weft yarns interwoven by a set of warp yarns. Most often, each of the warp yarns weaves the same contour pattern, only shifted by n shutes (weft yarns) from its neighboring warp yarn. After a number of such shifted warp yarns, the contour pattern has shifted a complete cycle and repeats the pattern (i.e. a complete pattern repeat). Typically, double layer fabrics are composed of 7, 8, 14, or 16 warp yarns. FIG. 1 is a schematic forming side view illustrating the surface weave pattern for a conventional double layer fabric. The fabric shown in FIG. 1 repeats every 8 warp yarns. In other words, the contour pattern of warp 1 would repeat as the next warp (9 not shown) above warp number 8, and vice versa. This pattern continues repeating (in the CD) across the fabric.
Many double layers fabrics incorporate a “paired warp” concept in which two warps yarns act together (i.e. as a pair) to effectively weave one unbroken contour in the top surface of the fabric. References describing fabrics with paired MD yarns include U.S. Pat. No. 4,605,585 (the “Johansson” patent) directed to a double layer fabric wherein as one warp yarn dips to the wear side layer the other warp yarn in the pair takes its place in the forming layer, U.S. Pat. No. 4,501,303 (the “Österberg” patent) where the warp yarn pairs are an integral part of the top layer but act as binding yarns on the bottom layer, U.S. Pat. No. 5,152,326 (the “Vöhringer” patent) where the paired warp yarns are vertically-stacked and integral to both the top and bottom layers, and U.S. Pat. No. 5,865,219 (the “Lee” patent) in which the warp yarn pairs produce a plain weave pattern in both the top and bottom layers.
Multi-layer fabrics, such as double or triple layer fabrics, may have unacceptable resistance to internal abrasion and/or the weave may loosen (i.e. the yarns may slide from their original positions within the pattern) during use. The present invention provides a fabric which overcomes such disadvantages.